High-strength fibers are used in many applications. For example, polymeric ropes are widely used in mooring and heavy lifting applications, including, for instance, oceanographic, marine, and offshore drilling applications. They are subjected to high tensile and bending stresses in use as well as a wide range of environmental challenges. These ropes are constructed in a variety of ways from various fiber types. For example, the ropes may be braided ropes, wire-lay ropes, or parallel strand ropes. Braided ropes are formed by braiding or plaiting bundle groups together as opposed to twisting them together. Wire-lay ropes are made in a similar manner as wire ropes, where each layer of twisted bundles is generally wound (laid) in the same direction about the center axis. Parallel strand ropes are an assemblage of bundle groups held together by a braided or extruded jacket.
Component fibers in ropes used in mooring and heavy lifting applications include high modulus and high strength fibers such as ultra high molecular weight polyethylene (UHMWPE) fibers. DYNEEMA® and SPECTRA® brand fibers are examples of such fibers. Liquid crystal polymer (LCP) fibers such as liquid crystal aromatic polyester sold under the tradename VECTRAN® are also used to construct such ropes. Para-aramid fibers, such as Kevlar® fiber, likewise, also have utility in such applications.
The service life of these ropes is compromised by one or more of three mechanisms. Fiber abrasion is one of the mechanisms. This abrasion could be fiber-to-fiber abrasion internally or external abrasion of the fibers against another object. The abrasion damages the fibers, thereby decreasing the life of the rope. LCP fibers are particularly susceptible to this failure mechanism. A second mechanism is another consequence of abrasion. As rope fibers abrade each other during use, such as when the rope is bent under tension against a pulley or drum, heat is generated. This internal heat severely weakens the fibers. The fibers are seen to exhibit accelerated elongation rates or to break (i.e., creep rupture) under load. The UHMWPE fibers suffer from this mode of failure. Another mechanism is a consequence of compression of the rope or parts of the rope where the rope is pulled taught over a pulley, drum, or other object.
Various solutions to address these problems have been explored. These attempts typically involve fiber material changes or construction changes. The use of new and stronger fibers is often examined as a way to improve rope life. One solution involves the utilization of multiple types of fibers in new configurations. That is, two or more types of fibers are combined to create a rope. The different type fibers can be combined in a specific manner so as to compensate for the shortcoming of each fiber type. An example of where a combination of two or more fibers can provide property benefits are improved resistance to creep and creep rupture (unlike a 100% UHMWPE rope) and improved resistance to self-abrasion (unlike a 100% LCP rope). All such ropes, however, still perform inadequately in some applications, failing due to one or more of the three above-mentioned mechanisms.
Rope performance is determined to a large extent by the design of the most fundamental building block used to construct the rope, the bundle of fibers. This bundle may include different types of fibers. Improving bundle life generally improves the life of the rope. The bundles have value in applications less demanding than the heavy-duty ropes described above. Such applications include lifting, bundling, securing, and the like. Attempts have been made to combine fiber materials in such repeated stress applications. For example, UHMWPE fibers and high strength fibers, such as LCP fibers, have been blended to create a large diameter rope with better abrasion resistance, but they are still not as effective as desired.
The abrasion resistance of ropes for elevators has been improved by utilizing high modulus synthetic fibers, impregnating one or more of the bundles with polytetrafluoroethylene (PTFE) dispersion, or coating the fibers with PTFE powder. Typically such coatings wear off relatively quickly. Providing a jacket to the exterior of a rope or the individual bundles has also been shown to improve the rope life. Jackets add weight, bulk, and stiffness to the rope, however.
Fiberglass and PTFE have been commingled in order to extend the life of fiberglass fibers. These fibers have been woven into fabrics. The resultant articles possess superior flex life and abrasion resistance compared to fiberglass fibers alone. Heat-meltable fluorine-containing resins have been combined with fibers, in particular with cotton-like material fibers. The resultant fiber has been used to create improved fabrics. PTFE fibers have been used in combination with other fibers in dental floss and other low-load applications, but not in repeated stress applications described herein.
In sum, none of the known attempts to improve the life of ropes or cable have provided sufficient durability in applications involving both bending and high tension. The ideal solution would benefit both heavy-duty ropes and smaller diameter configurations, such as bundles.